In a medium saucepan bring the potatoes, salt and vegetable stock to a boil.,Turn down heat and simmer for 4 to 5 minutes, remove from liquid and set aside.,In a small bowl, combine the grainy mustard, Dijon mustard, rice wine vinegar, horseradish, lemon juice and salt and pepper.,Whisk in oil.,In a medium saucepan, bring the milk, thyme, bay leaf and garlic to a boil.,Turn heat to low and poach black cod in liquid for 4 to 5 minutes.,Remove from heat and keep warm.,Blanch baby leeks in water, remove and set aside.,______,To serve: Place the room temperature potatoes with baby leeks on 4 plates.,Place salad on top of the potatoes and leeks.,Place warm smoked black cod on top of salad.,Top with small dollop caviar.